Assessment of infant/child growth is a key element of monitoring nutritional status and evaluating the impact of various programs. Recently, the WHO Expert Committee of "Physical Status: Uses and Interpretation of Anthropometry" raised serious concerns about several limitations of the current NCHS-WHO growth reference, among them the fact that growth of healthy, breastfed infants shows a different trajectory. The Committee recommended that a new international reference be developed. As a result, WHO has initiated a Multicentre Growth Reference Study to be conducted at six to eight sites in different continents. In each country, the study will include a longitudinal design from birth to 24 months and a cross-sectional component for children 18-59 months of age. The overlap between 18 and 24 months will be useful for merging the two curves. The study will be based on samples of mothers and infants who meet several selection criteria (e.g. high socioeconomic status); for the longitudinal component these will include adherence to infant feeding recommendations (exclusive or predominant breastfeeding for at least 4 months, introduction of appropriate complementary foods from 4-6 months, and partial breastfeeding thereafter until at least 12 months of age). The coordinators of the WHO Multicentre Growth Reference Study have identified our location (Davis, CA) as an ideal choice for a U.S. site, because of the high rate of breastfeeding in our community and our experience in conducting such studies. This proposal seeks funding for data collection for the longitudinal component at this site. For this component (N=180, of whom at least 70 will remain eligible and complete the entire 24 months), subjects will be recruited at the time of delivery and visited at 1,2,4 and 6 weeks, monthly from 2-12 mo and bimonthly from 12-24 mo. Anthropometry will include child weight, length, head and arm circumferences, and triceps and subscapular skinfold thickness, and parents' weight and height. Other data will be collected on socioeconomic and demographic characteristics, perinatal factors, morbidity, and dietary intake. Quality control will be assured through regular standardization sessions, random repetition of 5-10 percent of measurements, and regular inter-center coordination meetings. The data will be pooled by WHO to create a new growth reference, applicable to all infants and children internationally.